It takes time "to evolve into one LTE base"

I understand. Impractical though then. Why put AWS 1700 and 1900 on the same phone if eventually LTE will be the norm?? Just for now? If AT&T would have merged with T-Mobile, then there would have been no worries in that department. So now they have to start tweaking a whole line of phones for interoperability on a network that is going to evolve into one LTE base in the future? Maybe I am missing something here.

More time than the lifecycle of an iPhone which is a few years. If and when interoperability happens between carriers a LOT has to occur. READ THE ARTICLES. Technology has to produce a small enough radio chip that carries the LTE frequencies. Policy and/or law has to change where carriers allow roaming on each others networks. And the phonez capable of this have to be produced. And that's just to start....